In order to split a group reservation into separate reservations, right click it and choose Split Reservation option:
You will see a window with the guest list.
Choose the option Assign balance to each guest if you want each guest to pay his bill.
Or Assign balance to the selected reservation if you want one guest to pay for all the rest.
Each split booking will change the ID to XXXX-split-1, XXX-split-2 etc.
* Please note that you cannot split guests which are in a Double bed.